NASA's Artemis Moon mission was supposed to launch Monday. But it was delayed due to a problem one of the rocket engines. When it launches, it will be a giant step towards sending humans back to the moon. We're eager to know: What leaps in scientific knowledge will be gained?It's a question planted in our minds by the scientist Hal Walker, who led an experiment during the first lunar landing half a century ago. The goal: Beam a laser at the moon. This encore episode, Scientist in Residence Regina G. Barber talks to host Aaron Scott about the Lunar Laser Ranging experiment — and how shooting that laser helped us better understand one of Einstein's theories.Follow Aaron on Twitter @AaronScottNPR and Regina @ScienceRegina. Reach the show by beaming an email to ShortWave@NPR.org.